If by the term appetizer you are simply saying something before your prime rib dinner, then I would be serving a salad. You have a rich and heavy protein and starch item so you really want a light, fresh and lively appetizer. A salad can be fancied up if you with the addition of seared scallops, a few shrimps, mesquite grilled chicken, duck prosciutto, etc but the main part should be the actual greens themselves.

I am also doing roast rib of beef with roast potatoes and yorkshire puddings, cabbage and tons of gravy.I am going to make brie and cranberry puff pastry turnover for apps, very easy a blob of brie and a blob of cranberry sauce (not the jelly).